コモンローのスポンサーになれますよ。
離婚が成立しているほうが面倒がなくていいですが、1年以上別居していて婚姻が破綻していることを証明できればスポンサーになれます。
国内申請の審査マニュアルに次のような記述があります。
Sponsor or common-law partners still married to someone else
Persons who are married to third parties may be considered common-law partners provided their marriage has broken down and they have cohabited in a conjugal relationship with the commonlaw partner for at least one year.
Cohabitation with a common-law partner must have started after a physical separation from the spouse. Evidence of separation from the spouse may include:
• a separation agreement;
• a signed formal declaration that the marriage has ended and that the person has entered into a common-law relationship;
• a court order regarding custody of children; and
• documents removing the legally married spouse(s) from insurance policies or wills as beneficiaries.
In this situation, the legal spouse of the principal applicant cannot subsequently be sponsored as a member of the family class.
http://www.cic.gc.ca/manuals-guides/english/ip/ip08e.pdf
上記の証明書類がない場合も証明ができないわけではないですが、ちょっと面倒かもしれませんね。場合によっては、信頼できる弁護しかコンサルタントを雇うのも1つの方法だと思います。
国外申請の審査マニュアルの方にも同様の記述があります(
http://www.cic.gc.ca/manuals-guides/english/op/op02e.pdf、5.38. What happens if the common-law partner (principal applicant) is married to another person?)。
こちらのマニュアルは、コモンロー関係の証明にも役立ちますので、関連の箇所だけでも目を通しておかれるといいと思います。